1. Field of the Invention
The present invention relates generally to surface probing devices and methods for making the devices and, more particularly, to the design and fabrication of scanning probes.
2. Description of the Related Art
Scanning Probe Microscopy (SPM) is a general term used to describe a growing number of techniques that use a sharp probe to scan over a surface and measure some property of that surface. The major advantage of scanning probe microscopy is that the resolution of the microscopes is not limited by diffraction, as is the case when using a beam of light or electrons, but only by the size of the probe-sample interaction volume (e.g. point spread function) which can be as small as a few picometers. The resolution obtainable with this technique can resolve atoms, and true 3-D maps of surfaces are possible.
Scanning probe microscopy covers several related technologies for imaging and measuring surfaces on a fine scale, down to the level of molecules and groups of atoms. At the other end of the scale, a scan may cover a distance of over 100 micrometers in the x and y directions and 20 micrometers in the z direction. This is an enormous range, and the development of this technology is having a profound effect on many areas of science and engineering.
Some examples of SPM technologies include STM (scanning tunneling microscopy), AFM (atomic force microscopy), scanning thermal microscopy (STHMP), Magnetic Force Microscopy (MFM), Electrostatic Force Microscopy (EFM), and Scanning Capacitance Microscopy (SCM). SPM technologies share the concept of scanning an extremely sharp tip, typically about 1-100 nm radius of curvature, across the surface of an object. An SPM image of a surface at the nanometer scale can, for example, be obtained by mechanically moving the probe or the sample in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a function of position.
The scanning probe typically consists of a stylus, a cantilever arm and a mounting section. A scanning probe cantilever is a microscale bar, typically ranging in size from about 5 to about 500 micrometers, that bends when the associated stylus responds to a surface property on an object being scanned. The tip is usually a tapered silicon structure having a sharp apex that interacts with the surface being probed. The bottom or base of the tip is typically mounted on or otherwise integrated with a flexible cantilever, allowing the tip to follow the topography of the sample. When the tip moves in proximity to the investigated object, forces of interaction between the tip and the surface influence the movement of the cantilever. These movements are detected by selective sensors, and various types of interactions can be studied depending on the mechanics of the probe including dimensional and thermal properties. The tip is usually scanned relative to the sample, although sometimes the sample is scanned relative to the tip (e.g. the surface is scanned under the probe).
A surface probing device may also have an electrical connection from the stylus, through the cantilever arm, to external circuitry and/or a reflective coating on the cantilever arm. The electrical connection and the reflective coating can provide different ways to measure the response of the stylus apex to the surface being analyzed. A feedback mechanism is typically used to maintain the tip at a constant height above the sample during the scanning process. The tip can be modified in many ways in order to investigate different surface properties, and therefore the number of scanned probe techniques is constantly growing. For example, the tip may be coated with magnetic material or a conducting metal to image magnetic and electrical properties, respectively, of the sample using techniques known as Magnetic Force Microscopy (MFM) and Electrostatic Force Microscopy (EFM). Similarly, in Scanning Thermal Microscopy (STMP), a tip may have an integrated thermal sensing element to image thermal properties of the sample. The SPM tip-surface interaction can also be used to modify the sample to create small structures (nanolithography).
Scanning probes are typically manufactured out of silicon or silicon nitride materials. The silicon probes normally contain silicon tips and silicon cantilevers attached to a silicon substrate. The tips in the silicon probes are very sharp, typically less than about 10 nm radius of curvature. In addition to the tip sharpness, spring constant and frequency of the cantilever are important parameters to determine with respect to the application of a probe for a particular sample. For example, a biological sample might be better imaged with a soft cantilever (low spring constant, low frequency) and roughness on silicon wafer may be better imaged with a hard cantilever (high spring constant and high frequency). Since the cantilever thickness in silicon probes is usually controlled by an etching process, it is very difficult to control a fabricated cantilever thickness of less than about 1 μm of silicon with high percentage of yield and uniformity across the wafer.
On the other hand, the thickness of the cantilever in silicon nitride probes is controlled by a deposition process rather than an etching process, and a cantilever thickness of less than about 1 μm can be relatively easily achieved if the cantilever is fabricated from silicon nitride. U.S. Pat. No. 5,399,232 by Albrecht et al. describes fabrication silicon nitride cantilevers integrated with silicon nitride. The tip is molded out of an inverted pyramidal shape pit made in the silicon wafer; as a result, the tips suffer from poor sharpness. The radius of curvature of these tips is inherently large due to process limitations. U.S. Pat. Nos. 6,886,395 and 6,156,216 by Minnie and Manalis et al. respectively describe methods of manufacturing probing devices having silicon nitride cantilever with an integrated silicon tip. The silicon tip in these methods is covered with silicon nitride except the apex. These methods have limitation of controlling exposure of the tip to uncover the apex.
Some bio-applications of the tips require the surface of the tip to be chemically modified, a process known as functionalization. For fictionalization to take place, the surface must be substantially exposed, and it needs to be either hydrophobic or hydrophilic. Properties of silicon material are better controlled and understood compared to silicon nitride. The silicon nitride surface is typically hydrophilic in nature and therefore has limited application with respect to tip functionalization. In addition, the exposed surface area of the tip in probes having cantilevers with integrated silicon tips is generally too small for functionalization.
Silicon nitride probes typically also require a reflective coating because the silicon nitride cantilevers are transparent to the laser that is reflected off the probe for measurement purposes such as imaging. A thin layer of metal is therefore coated on the probe to make it opaque to the laser in order to get a reflection. These cantilevers bend on nanometer scale when imaging in fluid due to a bimorph effect. The degree of bending is of the same order of the magnitude as of features in many samples useful for nanotechnology applications, thereby limiting the resolution of the probe. For example, resolving some minute biological entities such as virus particles in liquid, particularly those having sub-nanometer features, is difficult with metal coated cantilevers.
Information relevant to attempts to address these problems can be found in U.S. Pat. Nos. 6,156,246; 6,886,395; 5,066,358; 6,016,693; 5,021,364; 5,399,232; 5,540,958; and 5546375 as well as U.S. Patent Application Nos. 2006/0254345; 2005/0279729; and 2005/0210967. However, each one of these references suffers from one or more of the following disadvantages:
(1) an optional layer of metal must be deposited on the probe to facilitate or improve reflection;
(2) tips fabricated from silicon nitride are not sharp enough for many applications;
(3) tips fabricated from silicon nitride are limited with respect to tip functionalization; and
(4) integrated silicon tips are difficult to fabricate, particularly with respect to controlling exposure of the apex.